Neuromuscular disease, COPD and obese hypoventilation patients often suffer from chronic respiratory failure. Patients with chronic respiratory failure need regular treatment of their respiratory failure at home. Hypoxemic patients are treated by oxygen therapy while treatment by Non Invasive Ventilation (NIV) helps bringing the high carbon dioxide blood gas level of hypercapnic patients back to an acceptable level. The efficacy of the ventilation is checked by measuring the base-line and the trends in the arterial oxygen and carbon dioxide levels during NIV.
Arterial blood gas measurements form the golden standard. Before starting NIV at home, patients stay at the hospital to optimize ventilator settings and monitor arterial blood gas values. Depending on disease severity and stability, patients have to return more or less regularly to the hospital for checks. A respiratory nurse can also visit the patient at home to check the ventilator and to install equipment that enables non-invasive monitoring of blood gas partial pressures. At home, levels are monitored typically during a night and data are stored together with ventilator and respiratory data for later analysis at the hospital.
The state of the art in non-invasive blood oxygenation monitoring is by measuring the arterial oxygen saturation, which relates to the partial oxygen pressure via the oxygen dissociation curve. Pulse oximetry (SpO2) is an optical method for non-invasive monitoring of arterial oxygen saturation in a patient and has become one of the most commonly used technologies in clinical practice. Pulse oximetry is a reasonably low cost technology and is easy to use. It is the preferred method for blood oxygenation monitoring at home.
The state of the art in non-invasive monitoring of the partial pressure of CO2 is by means of capnography or by transcutaneous CO2 (PtcCO2) monitoring. For intubated patients with a healthy lung system the end tidal CO2 (etCO2) value obtained by capnography offers a good indication of the arterial CO2 value. However, in case of non-invasive ventilation where air leaks between mask and face are usually present and the patients have severe respiratory diseases capnography is often not a reliable method. Moisture and dirt in the tubes connecting the patient's airway to ventilator can also influence the proper operation of a capnography sensor. In most hospitals a combination is used of capnography for trend monitoring and analysis of an arterial blood sample to obtain an occasional accurate value. Transcutaneous CO2 monitoring is not disrupted by air-leaks, respiratory diseases and moisture but requires trained personal to obtain reliable values and shows some inaccuracy due to variation in skin properties among adults. At home CO2 blood gas monitoring is less frequently used than oximetry despite its high relevance for patients receiving NIV.
The current transcutaneous CO2 sensor generally comprises a thermostatically controlled heater to increase blood perfusion and gas-permeability of the skin and to stabilize the response of the sensor, a fluid layer between skin and sensor membrane, gas-permeable membrane covering the sensor, an electrolyte solution between membrane and sensor, a sensor comprising an electrochemical pH sensor and reference electrode, and an algorithm to compensate for temperature effects and skin metabolism.
The sensor membrane and electrolyte solution have to be replaced typically every 2 weeks or before use when the sensor has been unused for a long time. This requires some expertise because a fixed amount of electrolyte solution needs to be applied to the sensor and no air bubbles may be present in the solution. A special tool is required to fix the membrane gas-tight on top of the sensor and electrolyte solution. Due to drift of the sensor, the sensor needs to be calibrated every 4 to 8 hours using a gas mixture from a reference gas cylinder. Before application of the sensor to the skin, a fixed amount of contact fluid has to be applied into a fixation ring that has been attached to the skin.
US2001/0034479 describes an optically based transcutaneous blood gas sensor having a reusable sensor head and a disposable sensing cap. The reusable sensor has to be placed on the disposable sensing cap and thereafter this unit has to be placed in a disposable receptacle that is attached to the patient's skin by means of an adhesive film.
WO 02/056023 A1 discloses an optical sensor for determining at least one parameter in a sample. The optical sensor comprises an indicator material responding to the parameter and having a short luminescence decay time and a reference material not responding to the parameter and having a long luminescence decay time. The optical sensor detects the measuring signal indicating the parameter to be detected on the basis of the luminescence responses of the indicator and the reference material that are commonly detected. The indicator and the reference material are immobilized on a common support. The layer facing the sample of the indicator material and of the reference material is covered by a layer that allows contact between the indicator material and the sample but is substantially impermeable to the light used for exciting the indicator and the reference material.